The invention relates to a mechanical seal arrangement comprising a seal member to be attached to a rotating machine part and provided with a slide ring. A static seal member is provided to be attached to a stationary machine part and provided with a slide ring. The seal members are fitted in place in such a manner that the slide faces of the slide rings are pressed axially against each other. Connections for supplying flushing liquid to the seal and for discharging it from the seal are provided. A secondary seal is arranged to surround the rotating machine part and to seal the gap between the rotating machine part and the static seal member.
These kinds of arrangements and seals are today widely used in various technical fields, e.g., for sealing pump shafts.
In mechanical seals to be flushed with a liquid, such as water, it is customary to suitably regulate the flow of the flushing water by means of a valve upstream in the flushing water supply conduit from where the flow of flushing water is introduced into the seal. No significant back pressure occurs on the discharge side, that is, on the side where the water flows out of the seal. The function of the flushing water introduced into the seal is to cool and lubricate the slide faces of the slide rings which are provided between static and rotating portions of the seal arrangement. The pressure of the water inside the seal is low, generally only equivalent to the flow resistance of the pipe system. Pressure within the seal should not rise too high, otherwise the slide faces of the seal are opened. This is due to the fact that the seals are usually so designed as to be closed by the pressure of the product to be pumped.
A problem with this type of seal is the secondary seal, that is, sealing between the rotating shaft and the static seal member. With greater flush water flow volumes in particular, piping resistances create such a high pressure within the seal that the secondary seal leaks.
For example, an annular gap has been used in the prior art in connection with the secondary seal in mechanical seals, whereby the secondary seal surrounds the shaft with a small clearance. A drawback of this prior art solution is its poor resistance to pressure or its great length and narrow clearance.
Another example is the use of a V-ring in connection with the secondary seal. A drawback of this solution is poor wear strength and excessively low pressure resistance, with the resultant risk of the slide faces being opened. A further problem is that the solution requires the use of flush water for operation.
Still another example of the prior art is the use of a radial lip seal called "stefa", in connection with the secondary seal. A drawback of this solution is its poor wear strength, which is affected by the high rates of rotation and the use of water as a lubricant. A further drawback is that the radial lip seal exposes the shaft to wear. Further, this solution provides too high a pressure resistance, with the resultant risk of the slide faces being opened. Still another drawback is that the seal requires, constant supply of water.